Among the conventionally-known centrifugal pumps are ones in which a rotation shaft rotatably projects into a volute and an impeller is mounted on the projecting rotation shaft. By rotation of the impeller, fluid present inside the volute is sent out from the volute. During rotation of the impeller, pushing force acts on water present forward in a traveling (rotating) direction of vanes of the impeller, so that local high pressure is developed on the front sides of the vanes. On the reverse sides of the vanes, on the other hand, pulling force acts on water, so that local low pressure is developed on the reverse sides. Thus, there would occur so-called “cavitation” involving decompression boiling that causes air bubbles. A technique addressing such cavitation is known, for example, from Japanese Patent Application Laid-open Publication No. SHO-64-73165 (hereinafter referred to as “relevant patent literature”).
According to the technique disclosed in the relevant patent literature, the impeller has a plurality of vanes extending radially outward from the rotation shaft and slightly slanted in the rotational direction of the rotation shaft. Each of the vanes has a generally straight shape with a distal end portion slanted on its front surface oriented forward in the rotational direction of the vane. With such a slanted surface, rapid changes in flows of water, and hence occurrence of cavitation, at the distal end portions of the vanes can be suppressed.
During self-priming operation in self-priming centrifugal pumps, the self-priming is performed by sucking in air through swirling flows developed at end portions of the impeller vanes. With the impeller disclosed in the relevant patent literature, however, water flows are smoothed by the slanted surfaces of the vanes, and thus, the swirling flows developed at the end portions of the impeller vanes would decrease. Consequently, an amount of air sucked in from the volute would decrease, which undesirably results in lowered self-priming performance of the pump.